The present invention relates, in general, to heterostructure field effect devices, and more particularly, to heterostructure field effect devices having refractory ohmic contacts directly on the channel layer.
III-V heterostructure field effect devices often use a low band gap, high mobility material, such as GaAs or InGaAs as a carrier channel. Furthermore, such devices often employ an aluminum containing barrier layer above the channel layer, such as AlGaAs or AlInAs. Recently, refractory ohmics such as NiGeW, NiInW and GeNiW have been used for source and drain contacts on III-V heterostructure field effect devices. The refractory ohmics are preferable to gold based materials which have been used in the past because the refractory ohmics form limited reaction contacts to semiconductors, and exhibit nonspiking interfaces with the semiconductor.
Unfortunately, refractory ohmics do not make particularly good ohmic contact to aluminum containing barrier layers which typically cover the channel layer of a III-V heterostructure field effect device. This problem is particularly severe when aluminum composition in the barrier layer is more than twenty percent. The problem is due to high reactivity of aluminum containing compounds, i.e. oxides are readily formed on the aluminum containing barrier layer. The oxides formed on the barrier layer seriously degrade contact resistance of the refractory ohmic, since they impede the reaction of the metal and the semiconductor.
Sometimes, a GaAs cap layer is formed over the aluminum containing barrier layer in order to prevent the formation of oxides. Given such a structure, the refractory ohmic may be formed on the GaAs cap layer. However, since the high aluminum mole fraction barrier layer beneath the cap layer inherently has relatively low doping, tunneling resistance increases significantly across the residual barrier layer, resulting in poor ohmics.
What is needed is a structure and method for making a structure which employs the advantageous refractory ohmics but avoids contact to a high aluminum composition barrier layer or GaAs cap layer. Such a structure and method would be particularly useful in the context of a III-V complementary heterostructure device wherein a high aluminum composition and GaAs cap are nevertheless required.